1. Field of the Invention
This invention concerns a potassium titanate powder used as additives to plastics, friction material, paints, lubricants, heat resistant materials, heat insulative materials and paper products and, more in particular, it relates to a potassium titanate powder with an importance being attached to configurational characteristics in view of sanitation.
2. Statement Related Art
Potassium titanate inherently has a shape of several xcexcm to several tens xcexcm length and 1 xcexcm or less diameter and generally referred to, for example, fibers, needles or whiskers in view of the shape. Potassium titanate is put to practical use and popularized mainly as reinforcing materials in the field of plastics, paints and frictional materials taking advantage of the fibrous shape thereof. However, the fibrous powder has a nature of bulkiness, poor fluidity and inconvenience in handling. In addition, the fibrous powder tends to cause dusting to result in a problem in view of working environment.
There has been a problem of carcinogenicity for asbestos and this is sometimes considered to be attributable to the fibrous shape thereof. According to the Stanton""s assumption for the fibrous material, fibers with a diameter of 0.25 xcexcm or less and a length of 8 xcexcm or more are highly tumorigenic. Industrially useful fiber materials are utilized under the regulation in view of the working environment. Those fibers having a diameter of 3 xcexcm or less, a length of 5 xcexcm or more and the length to diameter ratio of 3 or more are specified as respirable fibers by ILO (International Labor Organization). Such fibers are specified to be managed as fibrous dusts also by AIA (Asbestos International Association), DFG (Deutsche Forschungs Gemeinschaft).
It is an object of this invention to provide a potassium titanate powder of excellent configurational characteristic not having the shape and the dimension of the respirable fibers with a view point of safety.
In view of the above, the present inventor has already filed Japanese Patent Application Hei 11(1999)-103033, and the present invention concerns potassium titanate of higher safety.
This invention has been developed for attaining the foregoing object and provides a potassium titanate powder having a length of particles of less than 2 xcexcm. More preferably, the powder having a length of particles of less than 1 xcexcm. Further, potassium titanate particles of this invention has a length/breadth ratio (aspect ratio) of less than 5.
Further, the potassium titanate powder of this invention comprises 90% or more of particles with the length/breadth ratio of less than 2 and 97% or more of particles with the length/breadth ratio of less than 3 on the basis of the number of particles.
Potassium titanate of this invention is represented by the formula; K2O.nTiO2 (n=1-12).
The potassium titanate particles of this invention having the characteristic that the length is less than 2 xcexcm can be produced by firing a mixture of one or more of carbonate, hydroxide, nitrate and sulfate as a K source that forms K2O by firing and one or more of Ti sources such as TiO2 and titanium hydroxide, thereby forming particles with the length of less than 2 xcexcm, slurrying them with addition of water, adding an acid to leach out excess K+ ions to condition an aimed composition, dehydrating and then applying heat treatment.
The shape of the particles is determined by the first firing and does not change substantially in the succeeding steps, that is, slurrying, conditioning for composition and heat treatment.
When a mixture of K source and Ti source is fired particles of potassium titanate with the length of 2 xcexcm or more are usually formed by crystal growth, in which most of particles have a length/breadth ratio of 2 or more
Potassium titanate of this invention can be obtained by firing with the temperature for the first firing being at a low temperature near the lowest temperature at which reaction can proceed and for a firing time being restricted to such a short period that crystal growth does not proceed.
The blending ratio for the K source and the Ti source is within a molar ratio of 1 to 6 as TiO2/K2O. Particles with the length of 2 xcexcm or more and the length/breadth ratio of 2 or more increase if the molar ratio is less than 1 and the reaction less proceeds if the molar ratio is more than 6.
The firing temperature is within a range from 750 to 850xc2x0 C. The reaction less proceeds at a temperature lower than 750xc2x0 C., while the ratio of particles with the length of 2 xcexcm or more and the length/breadth ratio of 2 or more increases at a temperature higher than 850xc2x0 C.
For the firing method, a rotary kiln system is suitable. Firing method in a stationary state causes uneven temperature distribution tending to form particles with the length of 2 xcexcm or more and the length/breadth ratio of 2 or more. And a rotaly kiln system is preferable to complete the synthesis reaction at a short firing time.
Particles with the length of less than 2 xcexcm can also be formed by firing a mixture in which a small amount of one or more of carbonate, hydroxide, nitrate, sulfate and alkali halide of alkali metals such as Li, Na, Rb and Cs other than K is added to the K source and the Ti source. It is supposed that addition of the alkali metal other than K functions to lower the melting point of the mixture during firing tending to complete the synthesis reaction at a lower temperature thereby suppressing crystal growth to the fibrous shape.
The alkali metal slightly solid solubilizes into crystals of potassium titanate of this invention to form solid solution and can be used within a range satisfying an aimed X-ray diffraction pattern.
The thus obtained potassium titanate particles with the short length and low length/breadth ratio exhibit weak diffraction intensity and shows a diffraction line of wide full width at half maximum (FWHM) in the X-ray diffractiometry. This shows that the material is the low crystallinity and not in the fibrous shape.
Further, particles of flat and thin shape can be obtained by controlling the blending of the starting materials and the firing condition, when the flat particles are put to X-ray diffractiometry, it is observed that the diffraction intensity ratio for (h00) /(0k0) of the miller index shown by (hk1) is 3 or less as one of the features of the diffraction pattern. It is considered that this is attributable to the orientation of (0k0) crystal face. While existent fibrous potassium titanate extends in the direction of b-axis, potassium titanate of this invention is observed as thin and flat particles, and it is considered that it grows greatly in the two directions of a-axis and c-axis. Such potassium titanate particles are also free from fibrous property and suitable in view of safety.
In the potassium titanate particles of this invention, such a small amount of TiO2 unreacted with the K source and TiO2 formed by thermal decomposition of potassium titanate as detected qualitatively by X-ray diffractiometry may be present together unless they give problems in view of the application use. Further, they may contain secondary particles formed from cohesion of primary particles to some extent as ordinarily found in the powdery particles of industrial products.
Potassium titanate of this invention is a powder comprising particles. Alternatively, it is flat and has a property tending to orient. Accordingly, it forms a uniform sliding surface to provide excellent sliding characteristic as a brake material or the like. Further, it gives a high dimensional accuracy when blended with plastic materials and is suitable to precision molding products in view of improvement of the rigidity and provision of the surface smoothness.
Further, it is expected for the application uses in the fields by taking the advantage thereof for blending at higher ratio compared with fibrous potassium titanate.